Process and device for confining, retaining and sucking off fumes, dust or the like

ABSTRACT

Processes and devices for confining, retaining and sucking off vapors, fumes, dust or similar materials including dispersed or dissolved vapor particles in a fluid medium. In order to separate these pollution particles, a fluid boundary layer or front is generated by diverting a jet against a boundary surface. The curved jet forms a vortex flow retaining the particles and transports them to the suction surfaces. The process and device are especially useful for exhaust hoods in the kitchen field and in the field of clean rooms, furthermore, in those fields, where fluid media with different characteristics are to be separated, confined and suctioned off.

This invention refers to a process and a device for confining, retainingand drawing off vapors, fumes, dust or similar fluid materials,resulting from kitchen ovens, cooking places and industrial workingstations. However, the invention also can be used for retaining anddrawing off other fluid materials, such as solutions, dispersions orsuspensions. Especially, the invention refers to exhaust hoods to beused in the kitchen field and in the field of clean-air rooms.

Vapors, dust, fumes and the like are pollutants to be removed from airby drawing off the pollutants through a filter, for example a vaporhood. These materials often are included in very fast and turbulentstreams of air. A mere suction flow is not suitable for retaining suchstreams because it's intensity nor it's stability is sufficient todivert and draw off a turbulent flow. For this reason the volume of thedrawing off stream is chosen to be considerably larger than the volumestream of the pollutants, or a larger suction screen with high suctionpower is used.

DE 39 18 870 C2 discloses a method for improving the suction flow fieldof an exhaust hood. A downwardly directed free jet and a wall jetdirected towards the suction surface cooperate with each other andgenerate a frontal vortex. The flow field produces an aerodynamic wallaround the exhaust hood.

DE 42 03 916 C1 discloses a process for forming a blast flow accordingto DE 39 18 870 in such a manner that it results in a higher inherentstability and is formed as a helix, and passes the frontal vortex alongthe lateral sides of the exhaust hood. The drawback of both describedprocesses is the expensive structure of a twin slot nozzle forgenerating the frontal vortex and the wall jet, as well as the problemin diverting the frontal vortex at the edges of exhaust hoods.

DE 33 04 262 C2 discloses a recirculation hood forming an air curtainaround the lateral walls of an exhaust hood. As can be seen from theSchlieren photographs of this type of air curtain, no distinguishedfront is generated. Especially, this type of recirculation hood is to befurther developed and improved by the subject invention.

In the field of meteorology, the expression ‘front’ is the boundarybetween different air masses. A front is a strongly convergent flow areaon which extreme gradients, for example temperature or moisturegradients, preferably adjacent to boundary surfaces, such as the groundor a wall can occur. According to the invention, this type of front alsois generated as an area of flow between the vapor section and the blowout area of the exhaust hood.

It is an object of this invention to improve the suction flow field ofan exhaust hood for vapors, fumes and dust in such a manner that vapors,fumes, and/or dust are separated from ambient air, and a front isformed.

SUMMARY OF THE INVENTION

A blast jet exiting around the front edge of the hood is diverted into amovement extending towards the suction surface, and is transformed intoa vortex or a curved shearing flow or shearing layer. Ideally, a vortexcomprises a fixedly rotating nucleus surrounded by a shearing flow or ashearing layer. For generating a front it is vital that the shearingflow is able to build a convergent stream field generating a front ifthe stream hits a wall or a counterstream. With the invention frontvortexes, as well as vortex or shearing streams are generated, anddevices are proposed which build a front on the underside of the exhausthood in a more stable and more effective manner, and also generatehelical suction flows.

Diverting a jet for obtaining a curved vortex or shearing flow isobtained in different ways:

1. A direct suction effect acts upon a jet. The jet is blown out at thefront edge of the hood into the area below the hood and, by means of gapsuction formed within the deeper inner edge area of the hood, isdiverted towards the bottom surface of the hood. Optimum orientation ofthe jet depends upon the intensity and the distance of the edgesuctioning from the blast slot. Preferably, the jet is orientated in anangle of +/−30° to the vertical in order to obtain a positive generationof a front vortex and a front layer. The aperture of the suction slot isprovided towards the center of the hood. The exit aperture and suctionaperture in their most simple embodiments are separated from each otherby a straight surface, the distance depending on the radius ofcurvature. The suction rate is in the order of the exhaust rate and isapproximately 3-5 m/sec. In front of the suction slot, a trough can bearranged to act as a receiving trough and as a means for diverting thesuctioned free jet and the vapor elements pulled along with the freejet.

2. The jet diversion is caused by the action of the Coanda effect on awall jet over a curved surface or by blowing the jet in an inclineddirection over a plane surface. The suction effect causing a curvatureand acting upon a free jet also can be generated by a free jet byblowing the jet over a curved surface. Such jet adheres to the curvedsurface and is diverted up to 240°. This effect is known as theso-called Coanda effect and generates a vortex flow or a shearing flow.The curved surface takes the function of a vortex nucleus, either partlyor totally. If a break edge is provided within the curvature, a vortexcan be generated at this break edge. A jet is directed outwardly over acircular profile or a partial circular profile in a horizontal directionand generates a flow which at the bottom side of the hood is directedagainst the interior of the hood. For improving the adherence of a walljet on a curved surface boundary layer, suctioning can be providedwithin the area where the flow separates from the surface.

A further possibility to provide a jet with a curved direction is toblow the jet at an angle a in view of the exit direction towards aninclined plate, a correspondingly inclined profile or a curvature if thejet adheres to the plate at an angle of 0<α<50°. This is possible with aplate which is approached with the indicated range of angles. The jetadheres in a distance of 5-30% of the thickness of the blow out slotbehind the slot at an angle of 25°<α<30°.

Another way of diverting the jet is to blow it onto a straight surfacein a tangential direction, which means that α=0°, and the jet is a walljet. This plane surface is joined by a curvature or a profile in orderto generate a corresponding flow. If a half-circular, a circular segmenttype, a profiled or a similarly curved element is inserted between thevertical blast jet and the horizontal wall jet of a nozzle according toDE 39 18 870 C2, the effect of the process according to the inventionwill improve because the nucleus of the generated front vortex does notneed to be built up completely or partly. Therefore, a larger proportionof the jet can be transformed into a vortex flow for generating a front.

3. Another possibility of diverting the jet is to direct a free jetleaving the front edge of the hood against a profile in such a mannerthat the jet is diverted in the direction towards the bottom side of thehood and towards the interior of the hood so that a curved vortex orshearing flow is generated. Such diversion of the jet towards the bottomside of the hood is equivalent to the effect of a flat of an airplanewhich, at high angles of incidence, passes the approach flow towards theairfoil profile.

4. A fourth possibility of diverting the jet is to combine thegeneration of a front vortex or a front vortex type flow according tothe above possibilities mentioned in items 2 and 3 with edge suctioningaccording to item 1, whereby surface suctioning can be dispensed with.

If several fans are used within an exhaust hood, either all fans can beoperated in the suction mode, and part of the suction air can bediverted, or alternatively, separated fans for suctioning off or fansfor blowing out vapors are used. The former process lends itself to anoption if a single fan is used. When operating in the air outlet modethere is the disadvantage that the amount of air for the jet depends onthe resistance of the air outlet conduit. In this case, the relationbetween jet volume flow and air outlet flow dependent on the used airoutlet conduit is to be adjusted by throttles within the air outletchannel and the blast air channel so that this method preferably is usedfor hoods operating in the recirculation mode. The suctioned airflow isdivided into the blast jet and the recirculated jet. The recirculatedair, similar to commercial hoods operating in the recirculation mode,can be blown into the area above the hood.

According to the second version described above, throttles can bedispensed with because a differentiation is made between the suction fanand the fan for generating the jet and the front which is called thevortex fan. A vortex fan blows out via the blow-off slot. Thecorresponding volume flow is dependent on the apparatus. A vortex fan isable to remove air by suction, as well as through its surface filter asby means of edge suctioning or from the surroundings above the hood, anda suction fan can be operated with both suction modes.

The suction flow field can be improved by means of correspondingstructural designs. One possibility is to homogenize the steam. If thebasic shape of the hood is a circular segment, an ellipsoid segment or asimilarly curved shape, the front are at the wall connections of theexhaust hood only. A continuous ring-like shape without any lateralrestrictions and irregularities of the front vortex is especiallysuitable for suspended exhaust hoods. This is true for all suctioningprocesses, which operate based on vortex flow or a front vortex forgenerating a front along the forward edge of the hood.

For cornered or rectangular exhaust hoods operating with edgesuctioning, it is useful to partially interrupt the suction flow inorder to obtain U-shaped vortex-type interruptions and to increase thelength of the front. The width of said interruptions is approximatelytwo-fold up to twenty-fold the thickness of the suction slot, whereasthe length of the suction apertures is approximately two-fold up tothirty-fold the thickness of the suction slot. The length of theinterruptions and apertures along the suction edge can be the same sizeor can be of different size.

With exhaust hoods or similar suction hoods without edge suctioning, thestream directed towards the filter surface is structured by tongue-likeor wave-like formations of the suction surface. On locations where atongue is positioned closer to the edge of the hood, an area ofconvergence is formed, whereas on those locations where a gap isprovided between two adjacent tongues, an area of divergence is formedat the bottom side of the hood. A pair of longitudinal vortexes isassociated to each tongue. The pair from adjacent gaps at the bottomside of the hood rotate towards the tongue and the exhaust effect.

If a blast flow is used for an exhaust hood generating a front, saidflow can be formed by additionally corrugating the edge of the hood andthe blow out slot. This is done in such a manner that the flow beingdiverted at the front side of the hood is provided with a component tothe center line of the recess. The recesses or wave crests are areas ofconvergence, the wave troughs are areas of divergence below the hood.The results in longitudinal vortexes within the flow.

According to a special embodiment of an exhaust hood of the inventionusing the Coanda effect and a rectangular basic surface of the hood, itis useful to stagger the blow out aperture away from the front edge ofthe hood towards the interior (towards the center of the hood) in orderto restrict the suction effect of the jet below the extension to thefront half of the chamber underneath the exhaust hood. Compared with ablow out opening immediately at the front edge of the hood the suctioneffect of the jet is amplified in this manner. The distance is case of aspecial embodiment is approximately 50 mm. In general, it is sufficientto blow out at the front side of the hood only, whereby with a specialembodiment the blow out slot is 4-5 mm, the blow out rate is 2-3 m/secand the tube diameter is 38 mm. At the lateral restrictions of the tubecirculated by air, longitudinal vortexes are formed which suppress theremoval of the vapor at the lateral edges of the exhaust hood. In orderto obtain a satisfying effect of these longitudinal vortexes, thevortexes also are to be arranged beyond a shield. The end of the blowout slot and the buve, therefore, is to be spaced from the lateral edgesby about 50 mm. When operating in the recirculation mode, it is usefulto let that part of the recirculating air which is not blown over thecurvature, flow as slow as possible and over a large area. This exitlocation is spaced as far as possible from the front side of the hoodbecause this flow is able to exert a suction effect onto the vapor sothat the efficiency of the hood is considerably reduced.

According to a further special embodiment of the invention, the exhausthood is formed so that two or more blasts jets are each provided withmeans for generating a curved shearing flow, which operate parallel toeach other. A blast jet within the exhaust hood is divided into twoseparate jets, which overlap each other along their lateral area ofcurvature at the edge of the hood in such a manner that the outer curvedwall is shorter than the inner curved wall. Two shearing flows spacedfrom each other will be obtained.

A special embodiment of the invention refers to a Coanda vortex hood.The blow out aperture is shifted or is spaced from the front edge of thehood towards the rear side. This restricts the suction effect of the jetbelow the extension to the semi-space, and amplifies the suction effectof the jet compared with a blow out aperture exactly at the front edgeof the hood. In this case, it will be sufficient to blow out at thefront side of the hood only. At the lateral restrictions of the tubecirculated by air, longitudinal vortexes are formed, preventing thevapor from disappearing at the lateral edges of the hood. Withcomparable known systems, the longitudinal vortexes have been generatedby special diversion means. For an acceptable structure of longitudinalvortexes, it is important that they are formed below a shield. The endof the blow out slot and the tube, therefore, is to be spaced distantfrom the lateral edges. In the recirculation mode, it is useful to makethat part of the circulating air not blown out over the curvature exitas slow as possible and over a large area. The exit location is to bespaced as far as possible from the front edge of the hood because theflow can exert a suction effect onto the vapor or fumes which wouldreduce the efficiency of the hood.

A further embodiment of the invention refers to a combination of frontalvortexes with edge suctioning, whereby the effect of suctioning off willbe improved. With frontal vortex hoods operating with edge suctioningeffect, the experts differ between blast edges and suction edges of anexhaust hood. The blast edge is an edge for blowing off in order togenerate a frontal flow directed toward the suction off apertures. Asuction edge is an edge at which the air is removed by suction. Theedges of an exhaust hood can be blast edges, blast and suction edges,suction edges or merely lateral edges (without any function as blast orsuction edges).

The edge suction effect operates either with strip-like surface filtersat the edge or with a slot at the edge, whereby the filter is arrangedbehind the slot.

Often, exhaust hoods do not justify the expenditure to arrange means forgenerating a frontal vortex along the entire hood edges or along theentire periphery of the hood. In these cases, a hood edge or part of theperiphery of the hood will be provided with blow out apertures. The edgesuction effect preferably is designed so that along a gap a very highsuction speed substantially equivalent to the speed of the blast flow isgenerated. The channel is widened in order to keep the speed of air aslow as possible when passing through the filter. However, a wall-typesurface suction effect will be possible at the edges instead of a slotsuction effect.

Improving the flow at the corners or at the end of the blow out meansfor generating a front vortex according to the invention is obtained by:

a) profiling the blow out means,

b) boundary layer suction,

c) positioning of the suction surface in a proper manner,

d) using an adhering jet, and

e) using a vortex tube.

The blow out flow of a front vortex hood at the lateral restrictions ofthe blow out slots is no longer “quasi two-dimensional”. Experienceshows that the flow at this location no long adheres as well on thelower edge of the hood, and sometimes is directed downwardly. In orderto make the flow adhere as much as possible at the corners and toimprove the stability, the surface of the curvature to be passed by airis profiled inclined less towards the end of the blow out aperture sothat the tendency for the flow to separate caused by the shape iscontinuously decreased. This corresponds with the offset of a wing, theangle of adjustment of the profile of which decreases outwardly or theshape of the profile alters outwardly (geometrical and aerodynamicaloffset). In case of a freely profile body circulated by air, the offsetcan be made according to the profile of the wing.

With a further embodiment of the invention, a tube circulated by air isprovided. The outer profile is a straight extension of a tangent to thecurvature, whereas inside the blow out means the extension isincreasingly shortened. The transient to the tube is designed as smoothas possible.

According to another embodiment lateral suction apertures close to theends of the blow out means are provided for stabilizing the flowlaterally. Furthermore, at the ends of the blow out means, a boundarylayer suction effect can be provided.

Another alternative is to blow out a second wall jet, which inconnection with a tube, acts as an adhering jet. This is adequate to atwin jet principle. The tube is provided with an inlet for the air ofthe adhering jet laterally within the interior of the hood. Below thehood a slot is arranged, from which the adhering jet exits. Bypositioning the inlet an d the outlet openings as well by diversionmeans the adhering jet can be directed inwardly.

Extending the frontal vortex or the curved shearing flow by means of anadditionally general longitudinal vortex at the ends of the blow outmeans is a further alternative of the invention. Stabilizing the blastflow by off-setting, by boundary layer suction close to the suctionsurface or by an adhering jet, also can be used at other criticallocations of the blow out device.

Furthermore, the invention proposes a suction device designed as aso-called vortex tube, where a radial and an axial flow are continuouslymerged so that this flow is formed into a rotating jet when exiting.This flow is suitable as an extension of a blow out flow. A vortex flowcan be arranged at the outside of a tube around which the flow ispassed. The tube also forms the air supply for the vortex tube. The airfor the vortex tube also originates from the blast area of the hood andpasses through the aperture within the tube through the inlet into thevortex tube. The jet flowing out from the exit aperture is divertedtowards the suction surfaces. If the exit of the vortex tube is notcentrically formed within the truncated cone, it will be below thebottom of the hood. However, the vortex tube also can be arrangedsloping downwardly into the space below the bottom of the hood, and thetruncated cone used for converging the flow can extend into the requireddirection. The rotational sense of the frontal vortex and of thelongitudinal vortex is provided so that the longitudinal vortex forms anextension of the frontal vortex at the corners. A vortex tube issuitable for continuing the frontal flow structure laterally, if cornerexhaust hoods are used. However, it can also be used to semi ring-shapedhoods, whereby the hollow body, such as a tube, can change into a vortextube.

Further possibilities for making the flow at the lateral sides of theexhaust hood more stable are obtained by continuously decreasing thethickness of the blow out gap outwardly so that the relation of thethickness of the gap to the radius of curvature is decreased. Frompractical experiments with the Coanda effect, it si known that the angleof diversion of the flow is larger, the smaller the relation has beenchosen. A further method is to increase the radius of the circularprofile outwardly with constant thickness of the blow out slot. Theprofile or the way of blowing out air through the blow out device is tobe designed so that there will be a longer contact time of the flow.This principle also is met by profiling the blow out device as mentionedabove. Basically, the blow out device also is to be offset in a suitablemanner, for example, by profiling or according to the geometrical offsetof an airplane wing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is explained in connection with thedrawing with examples of embodiments. The drawings show:

FIG. 1 is a basic diagram of generating a front by means of a frontvortex,

FIG. 2 is a basic diagram of generating a front by means of a vortex orshearing flow,

FIG. 3 is a basic diagram of the front side of a hood with blast jet andedge suctioning,

FIG. 4 is a diagrammatic representation of an exhaust hood with blastjet, edge suctioning, suction trough and surface suctioning,

FIG. 5 is a diagrammatic representation of the front side of a hood withcurved blast jet guide and boundary layer suctioning,

FIG. 6 is a diagrammatic representation of the front side of a hood withinclined blast jet guide and with stalling edge,

FIG. 7 is a diagrammatic representation of the front side of a hood withblast jet guide on a vertical and joining curved surface,

FIG. 8 is a diagrammatic representation of an exhaust hood with curvedblast jet guide, suction trough, edge suctioning and suction ringchannel,

FIG. 8a is a top view of FIG. 8 along line A—A,

FIG. 9 is a hood system with common suction space for a vortex fan and asuction fan with free jet suctioning along a profile body,

FIGS. 10a, 10 b and 10 c are semi circular, circular and semi ellipticalbasic shapes of an exhaust hood, each with a surrounding front,

FIG. 11 is an exhaust hood with edge suctioning and interruptions withinthe suction gap,

FIG. 11a is a plan view of FIG. 11,

FIG. 12 is a diagram of a tongue-like suction surface for forming areasof convergence and divergence, FIG. 13 is a representation of the frontedge of the hood and the blow out slot with corrugations in lateralcross-section,

FIG. 14 is a schematic representation of an exhaust hood withCoanda-effect in lateral cross-section,

FIG. 15 is an exhaust hood with Coanda-effect in a frontalcross-section,

FIG. 16 is a schematic representation of the front edge of the hood withtwin blast jet in lateral cross-section,

FIG. 17 is a revised embodiment of a hood according to FIGS. 14 and 15,

FIGS. 18a-18 c are further embodiments of hoods with edge suctioning,

FIGS. 19a-19 c are representations of exhaust hoods of different designwith curved shearing flows for generating a front,

FIG. 20 is a basic diagram of an embodiment of a profile body formed asa tube, circulated by air,

FIG. 21 is a further embodiment of a tube with a second wall jet,

FIG. 22 is a different embodiment of a tube with a vortex tube, and

FIG. 23 is a basic representation of a vortex tube for a semi-circularhood.

DETAILED DESCRIPTION OF THE INVENTION

According to FIG. 1, a front 1 is generated by a front vortex 2 aroundan exhaust hood with a bottom side 8. In FIG. 2, the front 1 isgenerated by a curved shearing or a vortex flow 3. FIGS. 1 and 2 showwithin an exhaust hood system, the difference between a front vortex 2and a curved shearing or vortex flow 3, as it is obtained if air flowsover a curved surface 4. The schematic flow profiles 5 (FIG. 1) and 6(FIG. 2) depict the nucleus 48 of the front vortex 2 rotating fixedlyand a shearing layer 7 joining outwardly, and that if air flows over acurved surface 4, which in FIG. 2 is a circular profile with the sameradius as the nucleus 48 of the front vortex 2, a boundary layer 49 willbe obtained followed by a shearing layer 7 away from the wall, aroundwhich the air flows. The two flow areas 7 and 49 are shown separatedfrom each other in FIG. 2 by a dotted line. When cooperating with thebottom side of the hood, a convergent flow is obtained, which generatesa front 1. Front 1 is dynamic and is caused by a vortex or shearingflow.

With an exhaust hood according to FIG. 3, the generation of a frontvortex 2 and a front 1 with boundary suction is shown by use of thesuction slot 10. The front vortex 2 is generated by diverting a free jet9 exiting at the front side 13 of the hood. Profile 12 shows that thesuction flow 11 in front of the suction slot 10 joins the shearing layer7 of the front vortex 2. FIG. 4 shows an exhaust hood with the frontside 13 of the hood according to FIG. 3, but with an additional suctiontrough 50 and a surface filter 25 for sucking off vapor. Vapor, fumes orthe like are engaged by either the suction slot 10 of the edge suctioneffect and is sucked off by the edge filter 51, or is held at the bottomof the hood and is sucked off by a surface filter 25. The blast airflowis indicated by dotted lines 60, and the exiting circulating air by 26.The blow out slot 27 through which the blast air 60 leaves the hood isalso seen. If an exhaust hood has a fan 52 or several fans from whichthe blast air is branched-off. In the exhaust mode, the required blastvolume flow can be adjusted by means of throttles 32, 33 within theexhaust conduit 54 and within the blast channel 15. If this type ofexhaust hood is exclusively used for recirculation mode, adjustablethrottles 32, 33 can be dispensed with. The air sucked through filter 25either exists as circulating air 26 through one or more slots 58, or asblat air 60 through the exhaust slot 27. By correspondingly dimensioningthe slots 58 and 27 in a suitable manner, the relation betweencirculating air 60 and blast air 26 is determined.

According to the embodiment of FIG. 5, the blast air 15 flows from theblast channel along a curved surface 14 as an edge suctioning effect,and forms front 1. The curved surface 14 has apertures 16 which byboundary layer suctioning improve the adherence of the jet so that underthe effect of destabilizing vapor streams, larger diversions will bepossible.

With the embodiment according to FIG. 6, blast air is blown-out from theblast channel 15 over a plate 17 inclined in view of the blow outdirection at an angle α. The curved shearing or vortex flow generatedthereby is indicated by numeral 3. There is a break edge 18 generating abreak vortex 19 acting upon front 1.

A variation of the embodiment according to FIGS. 5 and 6 is shown inFIG. 7. In this embodiment, the blast air exits via a plane surface 53as a wall jet 20 at an angle α=0° from the blast channel and flows alonga curved surface 4 joining the channel, resulting in a curved shearingor vortex flow 3 directed against front 1.

A further variation of an exhaust hood according to the invention isdepicted in FIG. 8 having a surface suctioning effect and an edgesuctioning effect combined with a blow out effect along a curvature, ofor blowing at, a profile. A suction fan 23 suctions air from the vaporarea through a ring channel 22 with suction slot 10 over an edge filter51. Another fan 24 suctions air through a surface filter 25 in thecenter of the hood from the vapor area and blows the air through theblast channel 15 to the blow out slot 27. This embodiment of an exhausthood is especially suitable for sucking off oily vapors since the oilcan be deposited within a collecting channel 28. Fans 23 and 24 areprovided with a separate suck-off chamber 29, the space between vortexcasing 24 and filter 25, and the ring channel 22. As shown by sectionA—A in FIG. 8a, this exhaust hood has a substantially semi-circularshape.

With an exhaust hood according to FIG. 9, a curved vortex flow 3 isgenerated by sucking the blast air over a profile 21, such as a wingprofile, and is directed against a front, which restricts the vapor areaat the other side and draws in the air along a surface filter 25. Vortexfan 24 and suction fan 23 are supplied from a common suction space 30.If separate vortex fans are provided, as according to the embodiment ofFIG. 9, the blast volume flow is independent from the flow resistance ofthe exhaust conduit joining the connection 54.

With the basic designs of exhaust hoods according to FIGS. 10a, 10 b and10 c, the hood is a semi-circular hood 34, a circular hood 35 and asemi-ellipsoid hood 36. Each are able to generate a front, the schematicshape being designated by 1. An exhaust hood similar to the one shown inFIG. 8 is shown in FIGS. 11 and 11a. FIGS. 11 shows a rectangular hood,having interruptions 38 of the suction slot 10 of the edge suction.

FIG. 12 shows a surface filter 25 provided with tongues or wave crests40, resulting in a convergence 41 of the suction flow, as well asindentations or wave troughs 55, located between said wave crests, andresult in a divergence 42 of the suction flow.

The flow path caused by the corrugations of a curved front side 13 ofthe hood is shown in FIGS. 13. The latter shows the underside of a hood,whereas FIG. 13 shows a vertical cross-section of the front side 13 ofthe hood and the blast channel 15. The blow out flow 47 flowing throughthe blast channel 15 is reflected by the deflection 43 of the wave crest57 of the front side of the hood 13. It is directed towards the centerline 44 of the wave troughs so that along this line a convergence 41exists below the hood. Within the center lines 45 of the wave trough 46a divergence 42 is generated. The generated helical longitudinalvortexes 46 below the hood are schematically shown on the extension ofthe center lines of the wave crests.

The embodiment according to FIGS. 14 and 15 refers to an exhaust hoodwith the Coanda effect. The hood has a rectangular cross-section and,according to FIG. 14, operates as a circulation hood. The hood 61adjacent to the front side 62 is provided with an outlet opening for theblast air at the bottom 64 of the hood distant from the front edge or,alternatively, is offset rearwardly at a distance of approximately 50mm. The blow out gap 63 has a width of about 4-5 mm and is restrictedtowards the rear side by a tube 65 circulated by air, which according toa special embodiment, has a diameter of 38 mm. The blow out speed of theblast air for this embodiment is about 2-3 m/sec. Shifting the blow outgap 63 further away from the front edge of the hood restricts thesuction effect of the jet underneath the projection to half the space,and thereby amplifies the suction effect of the jet compared with theblow out aperture at the front edge of the hood. The exit of thecirculated air is shown as 66 in FIG. 14. Longitudinal vortexes 67, 68are generated at the lateral restrictions of the tube 65. These vortexessuppress a deflection of the vapor at the lateral edges of the hood. Fora proper design of the longitudinal vortexes, it is important the theyare arranged below a shield 69, 70. The end of the blow out slots 63,and therefore the tube 65, is to be located distant from the lateraledges, as shown in FIG. 15.

The twin jet exhaust hood shown in FIG. 16 has two blow out channels 71,72, which are separate from each other. They pass the blast jets 73, 74downwardly and inwardly, and generate a curved shearing or vortex flow.The two exit locations of the blow out channels are distant from eachother or are staggered in height.

FIG. 17 shows a revised embodiment of an exhaust hood with a Coandaeffect according to FIGS. 14 and 15. The blast edge is at the front sideof the lateral edges without an exhaust aperture. The hood 80 of FIG. 17shows a fan 81, a surface filter 82 in the center area, edge filters 83,as well as an edge suction effect with suction slots 84. The filterelements 82, 83 are provided on an extension of the blow out channelbehind the edge suction slots.

FIGS. 18a, 18 b and 18 c show different embodiments of Coanda vortexhoods with boundary suctioning effect in plan view, namely FIG. 18a withlateral edge suctioning, FIG. 18b with U-shaped edge suctioning and FIG.18c without central suctioning. The hood 85 is provided with a frontvortex generator 86, a center surface filter 87, edge filter 88 andsuction slots 89.

FIGS. 19a-19 c schematically show a series of developments of exhausthoods according to the invention using curved shearing flows forgenerating a front. FIG. 19a shows the basic use of the Coanda effect.FIG. 19c shows a twin-jet version using the Coanda-effect. Theembodiment of FIG. 19b shows a two-jet version using the Coanda-effect.Transforming the semi-circular element of FIG. 18b into a profileresults in a combination of a profile body circulated by air (accordingFIG.) 9 with a Coanda-effect according to FIG. 19c. A second jet caneither be provided along the entire exhaust length or at predeterminedlocations, at which the flow is to be in close contact. A combinationwith a profile according to FIG. 19c, against which the air is blast, iscalled a free jet, which after a short distance of flow becomes a walljet, when air flows around the profile. The common characteristic of thefront jet generators of FIG. 19 is that the flow is diverted by the“wall effect”. Using a second wall jet according to FIG. 19b stabilizesthe jet diversion so that the adherence of the jet at the bottom side ofthe hood is improved (see FIG. 21). The hood 90 is shown with a blow outchannel 21, suction slots 92, curved blast jet guide 93, surface filter94, Coanda-profile body 95, wing profile body 96 and twin blast channel97.

The embodiment according to FIG. 20 shows a tube circulated by air,which hits a surface. This tube is the flow-around body of a frontalvortex hood. The surface is the underside of an exhaust hood. The tubeis profiled towards the edge. The profile decreases outwardly, and isprofiled increasingly steeper until, in the interior, the tube becomesthe body circulated by air. The profile is provided at the outer side asa straight extension 101 of a tangent 102 to the curvature of the tube.The extension 101 is increasingly shortened towards the interior of theblow out device. The transient area 104 is the area in which thestraight profile joins the curvature of the tube; the surfaces 101, 102,103 restrict the body. The surface 103 is the extension of the bottomside of the hood.

According to the embodiment of FIG. 21, a second wall jet is blown outat the hood 105, acting as an adhering jet. This corresponds with thetwin jet principle according to FIG. 16 and FIG. 19. A tube 106 withinthe lateral sides of the interior of the hood is provided with an inlet107 for the air of the adhering jet. Below the hood 105, a slot 108 isprovided as the exit for the adhering jet. By positioning the inlet andoutlet openings 107, 108, as well as by positioning diversion means 109,such as air baffles in front of the exit, the adhering jet can bedirected inwardly. However, an extension of the frontal vortex or thecurved shearing flow also can be provided by means of an additionallygenerated longitudinal vortex at the ends of the blow out means.Stabilizing the blast flow by offsetting, boundary layer suctioning nearthe suction surface or by an adherence jet also can be arranged at othercritical locations of the blow out means.

FIGS. 22 shows a vortex tube 101 with a radial and an axial flowcontinuously combined, whereby this flow is changed into a rotating jetat the exit. This flow can be used as an extension of a blow out flow.Accordingly, a tube 110 is provided at the outer side of which a vortextube 110 joins. Tube 110, circulated by air, operates as the air supplyfor the vortex tube 110. The air for the vortex tube originates from theblast space 112 (which is the space above the bottom of the hood) andpasses through the aperture 113 within tube 111 and through the inlet114 into the vortex tube 110. Jet 115 leaving the exit aperture, andforming the jet with a longitudinal vortex generated by the vortex tube,preferably is directed towards the interior of the hood and towards thesuction surface.

In FIG. 22, the exit stream from the vortex tube 110 passesnon-concentrically into the frustum so that it leaves the hood below thebottom of the hood. The vortex tube also can be oriented downwardly inan inclined manner into the space below the bottom of the hood, and thefrustum which is used for converting the flow, can point in the requireddirection. The rotation al direction of the frontal vortex and thelongitudinal vortex is chosen so that the longitudinal vortex at thecorners forms an extension of the frontal vortex or the curved shearingflow generating the front. The vortex tube 110 is especially suitablefor extending the frontal flow structure at the lateral sides in case ofcornered hoods. The vortex tube also can be used with semi-ring shapedhoods, whereby the hollow body circulated by air, in general a tube,changes into a vortex tube. This is schematically shown in FIG. 23. Thecurved element 116 is the plan view on a curved tube circulated by air,which tube is a rounded body. Joining the ends of this rounded tube arevortex tubes. FIG. 23 is a view from the top onto the open hood. Thelongitudinal vortexes starting from the exit apertures of the vortextubes are visible through the suction aperture 117.

What is claimed is:
 1. A process for confining, retaining and suckingoff vapors or dust by a hood according to which the vapor or dust havinga vapor flow is sucked off from a vapor area by a suction fan creating asuction flow with filtering via air channels, a suction surface beingformed on the bottom surface of the hood, and within a lower front areaof the hood a blow out flow is generated from a blast channelcounteracting the vapor flow, said process including the followingsteps: a. diverting the blow-out flow into a vortex flow adjacent afront side of an exhaust hood, b. forming the blow-out flow by aninteraction with a front flow ahead of the suction surface, c. aneffective suction surface extending downwardly along part of thedistance from a working table, and d. a blow-out jet generating afrontal flow shielding the vapor by interacting with the bottom edge ofthe hood and the vortex flow transporting the vapor to the suctionsurface.
 2. The process according to claim 1, wherein fan air is blowninto the vapor space via a profiled area in an inclined direction sothat below the bottom of the hood a deflected shearing flow and a vaporconfining front as well the vortex flow transporting the vapor to thesuck off areas is generated.
 3. The process according to claim 1,wherein a free jet from the hood is blown against a profile surface andis deflected to generate a front confining the vapor below the hood andthe vortex flow retaining the vapor and passing it to the suctionsurfaces is generated.
 4. The process according to claim 1, wherein thevortex flow is combined with an edge suctioning effect.
 5. The processaccording to claim 1, where the blow out flow discharged from the hoodis passed between a vertical free jet exiting the front of the hood. 6.The process according to claim 1, wherein a boundary suctioning effectis provided at the edges of the hood which is formed as a slot suction,whereby a filter is arranged in a widening area of the air channel. 7.An exhaust hood for carrying through the process according to claim 6,wherein distant from the blow out flow exit, a suction slot is providedwithin the bottom of the hood, and the suction flow is directed so thatit passes the front towards the vapor flow.
 8. The exhaust hoodaccording to claim 7, wherein a suction trough is provided at the bottomof the hood joining the suction slot which is curved inwardly andupwardly, and forms a restriction of the air channel near the filter. 9.The exhaust hood according to claim 7, wherein an edge filter isassociated with the suction slot within the air channel.
 10. The processaccording to claim 1, wherein the blow out flow is crossed in order toobtain a more stable and close flow of the stream at the corners.
 11. Anexhaust hood for carrying through the process according to claim 10,wherein at the blow out flow exit a profile is provided within the pathof flow of an exiting free jet, said profile is approached so that thevortex flow and a front is generated.
 12. An exhaust hood for carryingthrough the process according to claim 10, wherein flow restrictions arearranged within the blast channel flow for adjusting the volume flow,and within the air channel passing the suction flow.
 13. The exhausthood for carrying through the process according to claim 10, wherein thefront side of the hood is provided as a curved blast channel, whichnarrows at its exit and having an inner restriction wall which is ofpart-circular cross-section, the blow out flow forms the vortex flowflowing along the outer side of the inner restriction wall and alsoforms the front, and the inner restriction wall joins a surface filter.14. The exhaust hood according to claim 13, wherein a part-circularconfining wall is provided with openings for boundary layer suction. 15.An exhaust hood for carrying through the process according to claim 10,wherein the front side of the hood is formed as a curved blast channel,which narrows at the exit and an inner restriction wall of which is adownwardly and inwardly inclined extending plane plate with a breakingedge joined by an inwardly curved surface, which joins the surfacefilter so that below the plate a curved shearing flow and at the curvedsurface a detach vortex is generated.
 16. An exhaust hood for carryingthrough the process according to claim 10, wherein the suction fan and ablast air fan are connected by a common suction space behind the filtersurfaces.
 17. An exhaust hood for carrying through the process accordingto claim 10, wherein the suction fan and a blast air fan are connectedwith separate suction spaces.
 18. The process according to claim 1,wherein the blow out flow at the corners is effected by boundary layersuctioning so that a more stable and closer adherence of the flow ofobtained.
 19. The process according to claim 1, wherein adjacent thecorners of the blow out flow, a suction off surface is positioned, whichresults in a more stable and closer adhering blow off flow.
 20. Theprocess according to claim 1, wherein adjacent the corners of the blowout flow, a second jet is blown off, to obtain a better adherence of theflow.
 21. The process according to claim 1, wherein vortex tubes arearranged at the corners of the blow out flow so that a longitudinalvortex generated by the vortex tubes provides the passing-on of the blowout flow and generates a vortex flow directed towards the suctionsurface in order to stabilize the stream.
 22. The process according toclaim 1, wherein the thickness of a blow off slot is reduced outwardlyfor obtaining a more stabilized and closer contact of the stream at thelateral sides of an exhaust hood.
 23. An exhaust hood for carryingthrough the process according to claim 1, characterized in that at thefront end of the hood means are provided for deflecting the blow outflow exiting from the hood, said means generate a flow, that the flow isdeflected so that a vortex flow moves the vapor to the suction surface,and that by deflecting the flow a front is generated, which shields thevapor area by cooperating with the bottom of the hood.
 24. The exhausthood according to claim 23, wherein a surface is provided whichtransforms the downwardly directed blow out flow into the vortex flowbelow the bottom of the hood for obtaining a front at the exit of theblow out flow on the side of the hood associated to the vapor area. 25.The exhaust hood according to claim 24, wherein the surface is ofcircular segment form.
 26. The exhaust hood according to claim 24,wherein the surface is a curved, profiled surface.
 27. The exhaust hoodaccording to claim 24, wherein the surface is an inclined, planar plate.28. The exhaust hood according to claim 24, wherein the surface is acombination of a straight plate and a inclined surface.
 29. The exhausthood according to claim 28, wherein the inner side of the blast channelis curved and joined by a substantially vertical plane surface forforming a wall jet, the plane surface joining a curved surface.
 30. Theexhaust hood according to claim 24, wherein the blast channel is curved,and decreases in diameter downwardly, and the air channel is formed as apartial ring and receives an edge filter.